OCR Text |
Show A somewhat surprising behaviour is exhibited by the reduced blast temperature flame F-22. This flame gave both the best initial burnout and the earliest peak temperature of the Elk Creek coal flames. Figs. 1C and ID show that the pyrolysis behaviour is very dependent on coal type. Both the initial burnout levels, (AD = 0.25) and the initial temperatures are a strong function of the coal type. The temperatures, at AD = 0.25, for the Elk Creek and Armco coals, which are both high volatile bituminous coals, were different by more than 600°C. The actual peak temperature for the Armco coal is not known because it exceeded the maximum temperature measurable with the suction pyrometer, but is certainly higher than 1850°C. Tne burnout for the oil flame, as calculated from the measured gas concentrations, is also plotted on this curve. The initially slow combustion rate of the oil as compared to the coal, can be related to the very poor mixing between the oil jet and blast air. The oil is initially in a concentrated low momentum liquid jet, which needs a relatively long time to vaporize. The coal particles are injected with a much higher momentum and consequently faster mixing with the hot blast, resulting in the very quick devolatilization rate observed for the bituminous coals. Summarizing the results of table 2 and fig. 1, some important parameters effecting the combustion of coal under these conditions can be isolated. - The coal type is important in that it determines the amount of potential volatile matter which can be expelled in the early section of the flame. In the very short time available for combustion in a blast furnace raceway this relatively quick devolatilization process can be extremely important. - A h_gh stoichiometric ratio (low coal rate) is important to bring oxygen quickly to the surface of the char and soot particles that remain after devolatilization. If the stoichiometric ratio is too low the majority of the solid combustion occurs downstream of 1.5 m. - As can be seen by comparing F-18 and F-26 in table 2, if the stoichiometric ratio is high then the use of a smaller average particle size is effective in improving the burnout within the first 1.5 ra, and specially within the first 1 m. 10-13 |